STRATIGRAPHY OF UPPER CLAIBORNE STRATA IN WESTERN TENNESSEE AND HYDROGEOLOGIC IMPLICATIONS

Recent sonic drilling and borehole studies and geologic mapping in western Tennessee have clarified the stratigraphy of upper Claiborne strata, especially that of the Eocene Cook Mountain and Cockfield formations. These formations form the upper confining unit for the Memphis aquifer, a regionally important source of high-quality groundwater. Because of limited coring and ambiguous geophysical log signals, the upper Claiborne formations in western Tennessee were commonly designated based on idealized thicknesses of interbedded sand, silt, clay and lignite. Borehole cores from sonic drilling demonstrate that the contact of the Memphis Sand with the overlying Cook Mountain Formation is conformable, with fine-grained fluvial- or distributary-channel sand and interbedded silt and clay abruptly grading upward to delta-front and pro-delta laminated mud and graded, rippled silty very fine-grained sand. Delta-front and pro-delta facies define the Cook Mountain Formation throughout western Tennessee and grade eastward to silty mud and sand truncated by overlying Pliocene and Pleistocene deposits. The contact of the Cook Mountain Formation with the overlying Cockfield Formation is disconformable, commonly above a pronounced paleosol beneath a coal or at the base of sandy incised fluvial-channel deposits. The Cockfield Formation is heterolithic, including fine-grained fluvial- and distributary-channel sand, delta-front and pro-delta mud and very fine-grained sand, and bioturbated bay mud and nearshore sand. At least 24 m of incision through the Cook Mountain Formation and fill by sandy Cockfield fluvial-channel deposits locally create semi-confined conditions in the underlying Memphis aquifer, which, along with a downward hydraulic gradient in the Memphis area, increase vulnerability to recharge of poor-quality water to the Memphis aquifer.